The present invention concerns a method of the kind defined in the preamble of Claim 1 to maintain the pressure at the desired level in high-pressure chambers during pressure treatments. The invention is especially but not exclusively applicable to the manufacture of synthetic diamonds.
The constitutional diagram for carbon and the phase curve concerning the constitutions of graphite and diamond are on the whole well known to-day and have proved very helpful in the attempts to manufacture synthetic diamonds and it is now possible to manufacture synthetic diamonds suitable for industrial use on the basis of graphite in high-pressure apparatuses which develop extremely high pressures during the manufacturing process.
Prior-art high-pressure apparatuses designed for diamond manufacture usually comprise an inner pressure chamber, a pressure-absorbing device surrounding the pressure chamber and one or several, usually two, pressure pistons which are displaceable in the direction towards the chamber centre while reducing the volume of the chamber for the purpose of compressing a graphite body disposed inside the chamber. The pressure exerted by the pressure pistons is not always transmitted directly to the graphite body but may be transmitted via an elastic sealing material, such as rubber or similar material. Another common sealing material is pyrophyllite which is an aluminium silicate material which is capable of forming a sealing medium when pressed into voids.
In theory it is possible to convert graphite into diamond by exposing the graphite to pressures and temperatures within the diamond-stable range of the carbon phase diagram in accordance with the theory expressed by Berman and Simon (Zeitschrift fer Electrochemie, 59, p. 355 1955). Later research (by e.g. ASEA and General Electric Company) has established the existence of both diamond-stable areas and areas with stable carbon of non-diamond character within the carbon phase area and contrary to findings by earlier scientists one has assumed lately that mere submission of carbon of non-diamond character to conditions of such a nature that it falls within the diamond-stable area of the phase diagram fails to meet the requirements necessary for conversion of carbon to diamond. To meet all requirements it is assumed in accordance with the Swedish Pat. No. 215 013 granted to General Electric Company that also a catalyst must be present. This thought is supported by Japanese scientists (see Swedish Pat. Nos. 328 856, 330 005, 338 760) but was later abandoned by General Electric Company (see Swedish Pat. Specification No. 333 137) which company now is said to have established that hexagonal shapes of diamond may be manufactured at high pressures and temperatures in the absence of catalysts. The last-mentioned Swedish Pat. Specification No. 333 137 thus suggests the introduction into a high-pressure/high-temperature apparatus of a quantity of graphite wherein the crystallite domains are comparatively large and perfect and the c-axes predominantly in aligned relationship in a certain direction, whereby the graphite is orientated in the apparatus in such a manner that the direction of the c-axes orientation becomes essentially aligned with the effect of the pressure force. By exposing the material to a variable high pressure which is at least as high as the pressure of the three-phase equilibrium point (the so-called tripple point) between solid diamond, solid graphite and liquid carbon, and by elevating the material temperature to at least 3000.degree. C., hexagonal diamonds are obtained in the graphite mass when the latter is allowed to resume the environmental pressure and temperature.
Following extensive studies of the carbon phase diagram one believes to have established with certainty the existence of one area where both graphite and metastable diamond may exist, of one area where diamond and metastable graphite exist, of one high-temperature area where only graphite exists, of one high-pressure area where diamond is the only existing phase, of one area of even higher pressure where carbon (diamond) exists in metallic form, and of one pressure/temperature area where only liquid phases exist.
Alongside with the studies of the carbon phase diagram, high-pressure apparatuses have been developed which largely have made these studies possible. These apparatuses may be divided into two main types. The apparatuses in accordance with the first main type consist in principle of a hollow pressure cylinder and two pressure pistons. The cylinder is in general surrounded by several concentric sections forming a band or girdle of a shrinkage fit consisting of continuous or sectional steel rings. The pressure pistons are inserted into the cylinder chamber forming the high-pressure chamber from each end of the pressure cylinder cavity, and a reaction mass (graphite when synthetic diamonds are to be manufactured) is positioned in the centre of this high-pressure chamber, surrounded by a concentric cover of pyrophyllite. The entire apparatus is placed in a hydraulic press and the pressure pistons are forced into the cylinder cavity towards one another. The contents in the centre will thus be compressed and part of the pyrophyllite will be forced out into the spaces between the pistons and the cylinder. Pyrophyllite possesses the characteristic that its internal friction increases considerably upon pressure increase. The pyrophyllite thus displaced into the spaces builds up a resistance against further displacement, thus forming a sealing rendering possible further compression of the rest of the contents in the high-pressure chamber. Electric current is used to increase the temperature and may be led directly through the reaction mass, if the latter consists of graphite.
Apparatuses of the second type are distinguished from the first-mentioned type primarily in that no sealings of pyrophyllite or equivalent substances are used. In the centre of the apparatus is arranged a steel cylinder containing two chambers of different size, the smaller one being a high-pressure chamber and the larger one arranged to provide a hydrostatic support pressure for the pressure pistons which are urged into the high-pressure chamber while compressing the contents thereof. A number of conical pistons are positioned around the cylinder, each one of said pistons fitting into its respective one of conical holes formed in an outer cylinder. The pressure is obtained in that the pistons (six as a rule), together with a block covering the inner cylinder, are forced into their respective conical hole in the outer cylinder. The inner cylinder including the high pressure chamber is compressed radially and axially. The material to be treated is compressed in the high-pressure chamber and the required heat is obtained through electric resistance members or through direct electrical heating.
A feature common to all these known methods and apparatuses is that the pressure exerted on the reaction material is to be brought to a certain value at a certain temperature and that the material is to be maintained at this pressure and at this temperature over a certain length of time. The lowest pressure is determined by the chosen working temperature and because it is necessary to maintain a comparatively high temperature to achieve the phase conversion within a reasonable time, it is also necessary to use a correspondingly high pressure. The effects of the volume reduction to which the reaction material is subjected during the conversion phase into diamond have, however, been neglected, which may be due to the fact that it has been considered possible to use sufficiently high pressures to remain at the right side (above) the equilibrium line of the phase diagram under all circumstances. The risk of falling below the critical pressure at the prevailing temperature when the reaction (phase conversion) is initiated is avoided in this manner, but the product obtained is unavoidably poorer from a quality point of view and it also becomes necessary to use equipment having an unnecessarily large pressure capacity, resulting in unnecessarily large energy consumption to generate the required pressure.